This invention relates broadly to ionization-chamber smoke detectors, or detectors of colloidally suspended particulates in a gas, and more particularly to an improved smoke-detection system utilizing one or more such detectors.
Referring to FIG. 1, smoke-detection systems of the ionization-chamber type commonly include two ionization chambers 1 and 3 which are connected in series across a d.c. voltage supply 5. Each chamber includes a pair of spaced electrodes 7 and 9 for establishing an electric field therebetween. The detector chamber 1 is open to atmosphere, whereas the reference chamber 3 contains air but is virtually sealed from atmosphere. As shown, each chamber is provided with at least one radioactive source 11 (chamber 1) and 11' (chamber 3) for ionizing air molecules therein, the resulting ions being attracted toward the chamber electrodes. As a result, a very small ionic current flows through the external circuit connecting the electrodes of the chamber. An increase in the number of visible or invisible particles in the air being sampled by the chamber 1 causes a decrease in its ionic current.
In the particular conventional detector shown in FIG. 1, the radiation source 11 for the atmospheric detector 1 is an alpha source of annular configuration. The source is mounted about the upper end of a support rod 6 as shown. For calibration purposes, a radiation absorber 10 in the form of a flanged cap is mounted to the upper end of the support rod. The cap is movable axially of the rod, and during calibration of the detector it is moved to a position where its flange intercepts and absorbs a selected fraction of the radiation which otherwise would penetrate the ionization region 14.
Whereas the measuring chamber 1 is operated in the ionically unsaturated state, the reference chamber 3 is operated in the saturated state and constitutes a constant-current device having a very high dynamic resistance. Thus, a decrease in the ionization current through the detector chamber 1 produces a relatively large voltage change at the junction 13 of the chambers. A gas triode 6 is connected across the chambers, with its starter electrode 17 connected to the junction 13 and its cathode connected to the negative electrode of detector 1. The triode normally is in the non-conductive state, but if the number of particles in the detector chamber increases to a so-called threshold value, the voltage across the chamber increases to a value exceeding the triggering, or breakdown, voltage for the cathode-to-starter-electrode portion of the triode. This initiates a cathode-to-starter current, which in turn initiates a relatively heavy discharge between the cathode and anode of the tube. This discharge energizes an alarm relay 25, which in turn energizes an alarm 22. A normally closed reset switch 23 is provided for opening the anode circuit, thus returning the triode to the non-conductive state and resetting the detector system. The ionization chambers 1, 3 and the triode commonly are designed as a compact assembly, or "detector head."
A smoke-detection system of the kind described above is subject to the disadvantage that its sensitivity to particles varies with ambient temperature. For example, tests conducted with a commercial detector head of the kind illustrated in FIG. 1 show that it incurs a total loss of sensitivity if the ambient temperature increases 10.degree. to 25.degree. F. above the value for which it was calibrated and that it generates false alarms if its temperature decreases more than 10.degree. F. below the calibrated value. The detector head can be re-calibrated for operation at a different temperature by manually adjusting the source cap 10, but this not only is time-consuming but also is impractical if frequent re-calibration is required.
Detectors of the kind described can be rendered relatively insensitive to changes in ambient temperature by means of electronic compensation circuits. Alternatively, the effect of temperature-induced variations can be offset by means of circuitry utilizing a computer, as described in copending co-assinged U.S. patent application Ser. No. 643,466, filed on Dec. 22, 1976. However, the prior art temperature-compensation techniques are not as simple and inexpensive as desired.